Over the past decade I've used or demo'd a variety of Pass products during my speaker/amp journey... Thiel 2.4s (Classe CAP-151, McIntosh MC402, Pass X250.5, Pass XA30.5... eventually settled on a Pass INT-30A), Coincident Super Eclipse IIIs (Pass INT-30A, First Watt SIT2... eventually settled on Coincident Frankenstein 300b Monos), & Coincident Pure Reference Extremes (Coincident Franks, Lyndgdorf TDAI-3400, Atma-Sphere M60 Monos, Pass Labs XA25... eventually settled on First Watt F4 Monos).
As pure current sources with no voltage gain, the F4s are clearly not for everyone/everyspeaker, but they excel on my PREs. As I have pointed out elsewhere, the 6moons summary description is pretty spot on.... "relaxed ultra resolution." They have a bit of the sweetness of my 300b SET amps, but both clarity and bass control that beats out all the other amps I've auditioned on these speakers. The soundstage is expansive, imaging is precise, and there is absolutely no harshness or grain... an important quality with the accuton ceramic tweeters on the PREs.
I got my first F4 (actually I built it) to serve as a zero-gain bass amp to augment my Coincident Franks, but unfortunately biamping on the PREs just kills their coherency (I've tried both active, with the Lygdorf, and passive, with the F4)... even with the Franks providing the F4's input signal from their speaker posts. When I finally ran the F4 alone I was shocked to find that it was better than all the other amps I'd tried with the PREs.
Previously, the Franks (currently up for sale) had won every shootout, but they simply don't have enough bass control for the PREs and I was forced to move on. Super happy that I stumbled onto this amazing speaker/amp synergy, however.
Thanks Nelson, for coming up for the design, and making it possible for people like me to build their own (along with help from the amazing community over at diyaudio) now that the amp is no longer being produced.